1. Field of the Invention
The present invention relates to a motor drive apparatus equipped with a friction brake apparatus for braking a motor with a friction plate.
2. Description of the Related Art
In a motor drive apparatus, use is conventionally made of a friction brake apparatus for braking a motor with a friction plate held between an armature and an end plate. FIGS. 3A and 3B are sectional views illustrating a structure of a general friction brake apparatus.
In a friction brake apparatus 100, a friction plate 111 is interposed between an armature 112 and an end plate 113. A hub 122 is spline-coupled to the friction plate 111 and is made integral with a shaft 121 of a motor by shrink fitting such that the friction plate 111 is also rotated in linkage with rotation of the shaft 121 of the motor. The end plate 113 and a spacer 117 are coupled together by a bolt 118, and the armature 112 is coupled to the spacer 117 so as to be movable in a direction toward and away from the friction plate 111. In a core 116, there are provided a spring 114 and a brake coil 115. In a state in which a brake coil voltage is not applied to the brake coil 115, the armature 112 is strongly pressed against the friction plate 111 due to an elastic force of the spring 114, and the friction plate 111 is held between the armature 112 and the end plate 113 and prevented from rotating. Consequently, the shaft 121 coupled to the friction plate 111 also becomes nonrotatable, and a state occurs in which braking is applied to the motor. In contrast, when a brake coil voltage is applied to the brake coil 115, an electromagnetic force is generated in the core 116 and overcomes the elastic force of the spring 114 pressing the armature 112 against the friction plate 111, whereby the armature 112 is attracted to the core 116 so that the friction plate 111 is released from contact with the armature 112 and the end plate 113. Consequently, the friction plate 111 and thus the shaft 121 become freely rotatable, and a state occurs in which the brake of the motor is released.
As described above, in the friction brake apparatus 100, the motor is braked by non-application of the brake coil voltage to the brake coil 115, and the brake of the motor is released by application of the brake coil voltage to the brake coil 115. Since the brake is applied due to friction between the friction plate 111 and the armature 112 and that between the friction plate 111 and the end plate 113, the friction plate 111 is worn each time the brake is applied, and consequently the distance between the armature 112 and the core 116 in a state in which the brake is released is gradually increased. When the friction plate 111 is excessively worn, the friction plate 111 becomes unable to be released from contact with the armature 112 and the end plate 113, in other words, the brake cannot be released unless a large attracting force is generated by applying a higher brake coil voltage to the brake coil 115. The state in which the brake cannot be released as above is generally recognized as a “state in which the life of the friction brake 100 is reached” or as a “state in which the friction brake apparatus 100 is in failure.”
In order to avoid a situation in which the brake cannot be released, it is also possible to use a method such as applying a high brake coil voltage to the brake coil 115 such that a sufficient attracting force can be obtained even when the friction plate 111 becomes thin. However, this method may not be desirable because power is consumed wastefully as the brake is released by application of a wastefully high brake coil voltage in a state in which there is a small amount of wear in the friction plate 111, which has only been used for a short period of time, of the friction brake apparatus 100.
For such reasons, in a friction brake apparatus, it is important to accurately grasp the degree of wear of the friction plate and thus the timing when the life of the friction plate is reached, thereby making it possible that the brake is released reliably and efficiently.
For example, as described in Japanese Patent Publication No. 3192817, a method is known in which a predetermined load is applied to a motor in a state in which the brake is normally released, and a motor current at this time is previously stored; and each time the brake is released, the predetermined load is applied to the motor and when a motor current at this time is larger than the previously stored motor current, it is determined that a brake failure has occurred. This method makes use of the property that it is necessary to flow a large current through the motor when it is attempted to forcibly drive the motor in a state in which the brake is not released.
Further, for example, as described in Japanese Laid-open Patent Publication No. 2015-466, a method is known in which when a motor does not rotate after a brake release is commanded, a voltage applied to the brake is increased, and when the motor rotates, it is determined that the brake is normally released.
Further, for example, as described in Japanese Laid-open Patent Publication No. 2002-303344, an apparatus is known in which a brake apparatus includes, as a device that can determine the life of a vehicle's brake element, a detection circuit that detects that a lining of the brake element is worn by a predetermined amount and a measurement circuit that measures a running distance of the vehicle, wherein the apparatus determines a life span until the thickness of the lining of the brake element reaches an allowable wear limit thickness from a residual thickness at a time when the lining of the brake element is worn by the predetermined amount and a running distance of the vehicle until the predetermined amount of wear is reached.
As described above, in the friction brake apparatus configured to cause the brake to be engaged by friction between the friction plate and the armature and that between the friction plate and the end plate, it is important to accurately grasp the wear of the friction plate since the friction plate is gradually worn as the number of times of braking is increased and when the friction plate is excessively worn, it becomes unable to release the brake.
However, according to the inventions described in Japanese Patent Publication No. 3192817 and Japanese Laid-open Patent Publication No. 2015-466, it is possible to detect life arrival (in other words, occurrence of a failure) due to wear of a friction plate in a friction brake apparatus, but it is not possible to predict a timing when the life of the friction brake apparatus comes to an end. For example, provided it is possible to predict the timing when the life of the friction brake apparatus comes to the end, it is efficient in that it is possible to perform maintenance work such as repair, replacement or the like during a time slot for which the friction brake apparatus is not used such as close of business time, but according to the invention described in Japanese Patent Publication No. 3192817, it is inefficient in that the maintenance work is performed after occurrence of a failure in the friction brake apparatus is detected. Further, according to the invention described in Japanese Patent Publication No. 3192817, since the brake coil voltage applied to the brake coil remains constant regardless of the deterioration state of the friction plate, particularly, failure detection is performed by application of an uneconomically high brake coil voltage even in a state in which the amount of wear of the friction plate is small due to the duration of use of the friction brake apparatus being short, and thus, undesirably, power is consumed wastefully. Further, according the invention described in Japanese Laid-open Patent Publication No. 2002-303344, in order to predict the life of the vehicle's brake element, it is required that a circuit for measuring a running distance of the vehicle corresponding to a thickness of a friction plate be separately provided in the brake apparatus, and hence there is a problem in that the number of parts is increased, which leads to an increase in size as well as an increase in cost of the apparatus.